10

6

Cl

17

1s22s22p63s23p5

10

7

Ar

18

1s22s22p63s23p6

10

8

Click in the picture on the right to start the clip of
the lecture.

How many valence electrons in each of the following?

Inner core electrons shield valence electrons from the nucleus. If we
are on a valence electron looking back at the nucleus, the inner core
electrons shield a portion of the nuclear charge from us. So the valence
electron does not feel the attraction of all of the protons in the nucleus,
but the attraction of an effective nuclear charge which is less than the
total charge on the nucleus. The effective nuclear charge is determined
by subtracting from the number of protons in the nucleus (Z), the number
of inner core (I.C.) electrons that shield the valence electron from the
nucleus.

Zeff = Z - I.C.

Click in the picture on the right to start the clip of
the lecture.

What is the effective nuclear charge experienced by the valence electrons
in each of the following?

So lets consider the valence electrons in magnesium and consider the
shielding effect of its inner core electrons and whether electrons in
the same shell shield the electron we are considering.

Click in the picture on the right to start the clip of
the lecture.

So here is a question to see whether the concept of effective nuclear
charge is clear.

Click in the picture on the right to start the clip of
the lecture.

Let's review what we've discussed and extend the idea of shielding and
effective nuclear charge. You have to be able to determine the effective
nuclear charge on any electron in an atom in the A groups.